Stringhalt is a neurological disorder whereby the horse exhibits involuntary, exaggerated flexion of the hock. Australian Stringhalt is one of three different forms of stringhalt that have been documented in horses.
It is caused by grazing pastures which contain certain plants that are considered toxic to horses; the most common being Hypochoeris radicata (more commonly known as Flatweed, False Dandelion or Catsear). Other weeds, including Taraxacum officinale (Common Dandelion) and Malva parviflora (Marshmallow, Mallow Weed) have also very occasionally been associated with Australian stringhalt.
Hypochaeris radicata (flatweed)
The condition typically occurs in horses that are grazing drought-stressed pastures. A lack of pasture means that these horses begin to consume weeds such as those mentioned above, as there is no alternate source of forage.
Whilst the exact cause is unknown, there is evidence to suggest that H. radicata plants produce toxic metabolites following chemical or climate induced stress, which, in turn, may cause peripheral nerve damage in horses. There does not appear to be any involvement of brain tissue or spinal cord.
Stringhalt can affect any breed, although taller horses such as draft horses and Thoroughbreds are more commonly represented.
Clinical signs
The condition may begin as a mild and very subtle “hitch” in the stride, but often progresses until the horse is severely affected. One or both hind limbs may be affected.
The altered gait is often more noticeable when backing and turning the horse, making regular care such as hoof trimming and transporting very difficult for affected horses.
In addition to nerves in the hind leg, long nerves elsewhere in the body may also be affected. This includes the nerve that controls the larynx, causing laryngeal paralysis (“roaring”), which may be seen in affected horses.
An important point is that the onset of symptoms may not be evident for 1 to 3 weeks after removal from affected paddocks.
Differential Diagnosis of Australian Stringhalt include;
- Shivers
- Fibrotic myopathy
- Upward Fixation of the Patella
- Conditions affecting the brain or spinal cord
- Hind limb pain or irritation (Some horses with a hoof abscess may display a stringhalt like action.
Treatment and Recovery
Phenytoin (an anti-seizure medication), muscle relaxants and vitamins B and E have been used to reduce signs of the condition, with varied results. The most important action is to remove horses from contaminated pastures and to provide ample access to appropriate forage. Whilst the average recovery period is 6-12 months, it may take anywhere from a few days to 18 months for a horse to return to normal function. Unfortunately, some horses may never fully recover from the condition.
Prevention
Be aware of the weeds and plant that are known to be toxic to horses Do regular ‘pasture checks’ to assess for the presence of potentially poisonous plants Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival to avoid contamination of pasture. Ensure adequate roughage/feed supplementation.